The invention relates to a method for operating an internal combustion engine with an exhaust gas cleaning system comprising a SCR catalyst.
Internal combustion engines with an exhaust gas cleaning system comprising a SCR catalyst generally require that the SCR catalyst reaches its operating temperature necessary for nitrogen reduction as quickly as possible. For this, German Patent document DE 197 49 400 A1 discloses determining an efficiency for the SCR catalyst and changing certain internal combustion engine operation variables when it falls below a predetermined value in such a manner that the nitrogen oxide content in the untreated exhaust gas is reduced and the exhaust gas temperature and the temperature of the SCR catalyst is thus increased. However, a heating of the SCR catalyst, particularly in connection with a cold start or warm-up of the internal combustion engine, can lead to an undesirably high emission of particularly damaging nitrogen dioxide (NO2) even with a tolerable value for the total emission of nitrogen oxides (NOx).
Exemplary embodiments of the present invention provide a method for operating an internal combustion engine with an exhaust gas cleaning system comprising a SCR catalyst, where a discharge of NO2 to the environment, particularly in connection with a cold start or a warm-up of the internal combustion engine, is reliably limited to tolerable low values.
With the method according to the invention for operating an internal combustion engine with an exhaust gas cleaning system comprising a SCR catalyst, a nitrogen oxide storage amount of nitrogen oxides stored in the SCR catalyst is estimated, and if the estimate results in the nitrogen oxide storage amount exceeding a predefinable nitrogen oxide storage amount limit value the internal combustion engine is operated with a cold start engine operation method with predefineable values for predefineable internal combustion engine operation variables. The specific cold start engine operation method provided in the SCR catalyst when a nitrogen oxide storage amount exceeds the nitrogen oxide storage amount limit value, however, preferably does not find a use or at the most a use in a changed form, if, or as long as the nitrogen oxide storage amount limit value is fallen below. An excess fuel consumption can thereby be avoided, which typically adjusts with the provided specific cold start engine operation method, if this is not necessary from the view of the NOx emission which is viewed to be particularly critical.
The inventors have recognized that an undesirably high emission of NO2 is caused in the first instance by an amount of nitrogen oxides (NOx) stored in the SCR catalyst that is too high. The stored NOx can be present in physisorbed or chemisorbed form or also in a chemically bound form, for example as a nitrous acid. It was noticed surprisingly that NOx stored in the SCR catalyst desorbs mainly as NO2 with a heating of the SCR catalyst, whereas the NO part of the desorbed NOx is considerably lower. Due to the increased harmfulness of NO2 compared to NO, it is thus particularly desired to limit the NO2 emission. By means of the estimate of the nitrogen oxide storage amount carried out according to the invention, the risk of an undesired high NO2 desorption due to a heating of the SCR catalyst in connection with cold start or a warm-up of the internal combustion engine can also be estimated. If the nitrogen oxide storage amount exceeds the critical nitrogen oxide storage amount limit value, the cold start engine operation method according to the invention is activated. A heating of the SCR catalyst thereby takes place so early that the NO2 emission is limited to predefineable values. On the other hand, the cold start engine operation method according to the invention permits adjustment of a heating speed of the SCR catalyst in a defined manner, so that the NO2 emission is limited to predefineable values.
In an arrangement of the invention, values of internal combustion operation variables are preset with an activated cold start engine operation method in such a manner that the SCR catalyst is heated by the exhaust gases emitted by the internal combustion engine in such a manner that a predefineable desorption rate value for a rate of nitrogen dioxide desorbing from the SCR catalyst resulting due to the heating of the SCR catalyst or a predefineable maximum concentration of NOx nitrogen dioxide in the exhaust gas emitted to the environment is fallen below. It was noticed that a fast heating of a SCR catalyst loaded with stored nitrogen oxides can result in a quickly increasing desorption of NO2, that is, a high desorption rate. This is particularly the case if a desorption temperature region of typically +10° C. to +60° C. is reached or passed through during the heating. Depending on the height of the nitrogen oxide storage amount, a greater or lower high maximum concentration of NO2 in the exhaust gas emitted to the environment results therefrom. It is thus particularly preferred if a predefineable heating gradient maximum value for a heating gradient of the SCR catalyst is fallen below with a heating of the SCR catalyst in a further arrangement of the invention. By means of the values of internal combustion engine operation variables adjusted in dependence on the nitrogen storage amount and/or the temperature of the SCR catalyst, the heating gradient, and thus the desorption rate value or the NO2 maximum concentration, can be influenced in a defined manner and preset or predefineable limit values can be fallen below in a reliable manner. For example, by correspondingly adjusted values of internal combustion engine operation variables, a comparatively slow heating of less than about 10° C. per min, particularly in a temperature interval of −20° C. to +40° C., a soft NO2 desorption can be achieved, where critical NO2 peak concentrations are avoided. The adjustment of a low heating gradient is particularly advantageous if the SCR catalyst has a temperature just below or within the desorption temperature region. It is particularly advantageous, starting with low temperatures of the exhaust gas cleaning component, that is, less than 0° C., particularly less than −20° C., to initially adjust a high heating gradient of about 20° C./min or more in a first heating step. The exposition duration of the SCR catalyst in the temperature range critical for NOx storage is thereby shortened, the further NOx storage is largely avoided and the nitrogen oxide storage amount is thus limited. If the SCR catalyst has reached a temperature just below the desorption temperature region, that is, a temperature lying below about 10° C., a low heating gradient below the heating gradient maximum value is adjusted. It is thereby advantageous to adjust the heating gradient in dependence on temperature, particularly with increasing temperature.
In a further arrangement of the invention, the estimate of the nitrogen oxide storage amount is based on an operation duration of the internal combustion engine with a temperature continuously falling below a predefineable particularly first threshold temperature for the SCR catalyst. Longer low load operation times at low temperatures have proved to be particularly critical. If, for example, the internal combustion engine is operated for a longer time in the idle run below a catalyst-specific threshold temperature of typically about 30° C., the emitted nitrogen oxides in the SCR catalyst enrich in an increasing manner. With multiple successive cold start and/or warm-up processes, where the SCR catalyst continuously has temperatures, where a storing of NOx can take place, the respectively stored NOx amounts accumulate. With a following, particularly fast heating due to an increasing internal combustion engine load, an undesired high NOx desorption can thus result. According to the invention, this is met in that the nitrogen oxide storage amount over the operation time is estimated with NOx storage in an integrating manner. The cold start engine operation method can already be initiated already before reaching a critical integral nitrogen oxide amount and a defined heating of the SCR catalyst can be achieved. For estimating the nitrogen oxide storage amount, one preferably refers back to stored emission characteristic fields of the internal combustion engine and corresponding adsorption characteristic lines. An online calculation, based on an adsorption and desorption model, can also be provided for the SCR catalyst.
In a further arrangement of the invention, a fraction of a nitrogen oxide storage capacity of the SCR catalyst is provided as the nitrogen oxide storage amount limit value. The nitrogen oxide storage capacity that can maximally be stored by the SCR catalyst is typically highly dependent on temperature and also depends on the type and/or ageing state of the SCR catalyst. It is conveniently determined empirically beforehand and is stored in a control device. An ageing dependence can thereby be considered in addition to a temperature dependence. For an SCR catalyst of the iron or zeolite type provided preferably, the nitrogen storage capacity is typically in the region of 1 g to 30 g per I catalyst volume at low temperatures of 0° C. and less. By means of the orientation of the nitrogen oxide storage amount limit value provided according to the invention at the nitrogen oxide storage capacity determined in such a manner, an undesired high load of the SCR catalyst is avoided. It is thereby particularly advantageous, if the fraction of the nitrogen oxide storage capacity of the SCR catalyst is provided in dependence on temperature, particularly decreasing with decreasing temperature of the SCR catalyst.
In a further arrangement of the invention, the cold start engine operation method is deactivated after reaching a predefineable second threshold temperature for the SCR catalyst. The inventors have recognized that a storage of nitrogen oxides in the SCR catalyst above a typically catalyst-specific threshold temperature is low or even negligible, wherein possibly stored nitrogen oxides can already desorb below this temperature at least approximately. If the cold start engine operation method is deactivated as fast as possible after reaching the threshold temperature, an excess fuel consumption is thereby also avoided or at least limited. If the engine load required by the user exceeds a predefineable minimum value, where it is ensured that a further heating or at least no cooling takes place, the cold start engine operation method is preferably deactivated immediately after reaching the threshold temperature. Alternatively, it can be provided to keep this activated for a certain time. It is particularly advantageous in this connection to apply the exact time of the deactivation in dependence on the engine load. It can naturally also be provided to sense a NO2 desorption in a sensory manner and to deactivate the activated cold start engine operation method by terminating some of all measures taken thereby after exceeding a detected NO2 desorption maximum.
In a further arrangement of the invention, a multiple injection of fuel into one or several cylinder combustion chambers of the internal combustion engine is carried out with an activated cold start engine operation method, which comprises a first pilot injection, a second pilot injection following the first pilot injection and a main injection following the second pilot injection within a work cycle of the respective cylinder. It can thereby be provided to divide the first and/or the second pilot injection into two individual injections following each other quickly. By means of the at least two pilot injections preceding the main injection, an ignition of the injected fuel is also enabled with low engine temperatures below the freezing point. Preferably, a comparatively low fuel amount of about 20% or less with regard to the main injection amount is injected in the first or in the second pilot injection. In this manner, an ignition even at very low surrounding or engine temperatures of minus 20° C. or less is enabled. Due to the low pilot injection amount, a temperature decrease caused by evaporation is at least reduced and an ignition of the homogenized pilot injection amount is improved.
It is particularly preferred in a further arrangement of the invention if the first pilot injection takes place in a crankshaft angle region of larger than 20 degrees before an upper dead center in the compression cycle of the respective cylinder. Typically, the temperature in the cylinder is too low for a conventional diffusion combustion at low temperatures of minus 20° C. or less. With the early pilot injection according to the invention, a homogenization of the mixture is enabled, whereby the ignitibility is improved. A combustion conversion of the first pilot injection takes place with a corresponding ignition delay, which leads to an increase of the temperature level in the cylinder. The fuel amount introduced with the second pilot injection can thus evaporate quickly and also ignite. A HC emission typically coinciding with the homogeneous or partial homogeneous combustion has thereby been shown to be advantageous with regard to a reduction of the nitrogen oxide amount and the NO2 desorption amount. Even though the connections are not cleared up completely, it is assumed that a NOx storage is reduced by HC present with the NOx in the exhaust gas by blocking adsorption spaces. On the other hand, a reduction of gaseous and/or stored NO2 and/or NO appears to take place, whereby the NO2 desorption amount is also reduced. It can thus be provided to carry out a HC enrichment of the exhaust gas in a defined manner in parallel to the active cold start engine operation method and/or with a starting NO2 desorption. An adjustment of a predefineable HC target concentration in the region of 80 ppm to 3000 ppm, particularly preferred in the region of 200 ppm to 1000 ppm, can thereby particularly be made. For this, the injection amount of the first and/or second pilot injection is increased correspondingly. An addition of HC into the exhaust gas outside the engine, for example by a fuel addition unit can naturally also be provided for this.
In a further arrangement of the invention, the second pilot injection takes place at a time after the start of a conversion of fuel injected by the first pilot injection. By means of the choice of the time for the second pilot injection according to the invention, the combustion progress for the fuel of the second pilot injection and the following main injection is improved.
A further improvement of the combustion progress is enabled, if the main injection takes place at a time after the start of a conversion releasing heat by of fuel injected by the second pilot injection in a further arrangement of the invention. A safe ignition is thereby ensured even with very low temperatures. In this manner, HC emissions can also be kept comparatively low even with very low outer temperatures and a defined heating of the exhaust gas cleaning system is enabled. The main injection typically only takes place behind the upper dead center of the compression cycle, particularly only after about 10 degrees crank angle behind the upper dead center. A late combustion position or a late position of the combustion center of mass results thereby. This enables a safe ignition and a defined and quick heating of the exhaust gas cleaning system and thus the preferably provided zeolite-containing SCR catalyst. A NO formation caused by combustion is additionally reduced.
In a further arrangement of the invention, the implementation of the cold engine operation method takes place in a predefineable low load region of the internal combustion engine and the cold engine operation is deactivated with an internal combustion engine load above the low load region. After the deactivation of the of the cold start engine operation method, a combustion method with dominating diffusion combustion is preferably adjusted.
Advantageous embodiments of the invention are illustrated in the drawings and are described in the following. It is obvious that the above-mentioned characteristics and which still will be explained in the following cannot only be used in the respectively given combination but also in other combinations or on their own without leaving the scope of the present invention.